The present invention is directed to devices for cutting web material such as paper. It is intended particularly, but not exclusively, for transversely cutting sheets from continuous rolls of paper.
In some printers and plotters, paper is fed to the print mechanism from a continuous roll. Such a printer may be of the type illustrated generally in FIG. 1, in which a paper web 10 is fed from a supply roll 12 to a print station 14, where it is imprinted in accordance with signals from a control circuit 16. A take-up roll 18 draws the imprinted paper 20 from the print station 14 in response to signals from the control circuitry 16. At appropriate times, the continuous paper is cut at a cutting station 22, typically when a completed document has been printed. The FIG. 1 illustration suggests a cutting station of the guillotine type, in which a knife edge is driven perpendicularly to the paper to cut the entire sheet at one time. This requires a certain coordination between the take-up roll 18 and the cutting station 22 in order to insure that the paper 20 has stopped when the cutting operation is performed.
An example of a different type of a cutting mechanism is depicted in FIGS. 2-4, in which a stationary central cylinder 24 provides front and rear axially extending openings 26 and 28 through which paper can extend, guided by a paper guide 30 interior to the central cylinder 24. Mounted coaxial with the stationary cylinder 24 and exterior to it is a rotatable cylinder 32, which has front and rear openings 34 and 36. The outer cylinder 32 is biased to the position illustrated in FIG. 2, in which its front and rear openings 34 and 36 are disposed in registration with the front and rear openings 26 and 28 of the inner cylinder so that paper can extend through openings in both cylinders. To cut the paper, the outer cylinder 32 is rotated clockwise in FIG. 2 so that the upper edge 38 (FIG. 4) of the outer-cylinder opening 34 moves across the lower edge 40 of the inner-cylinder front opening 26 to sever the paper 42. Preferably, the outer-cylinder upper edge 38 forms a slight axial angle with the inner-cylinder lower edge 40 so that the cutting point moves across the openings--i.e., the paper is not cut along its entire width simultaneously. This clearly provides an advantage over the guillotine-type mechanism. However, it is slightly elaborate mechanically, and this is not entirely desirable in lower-cost printers and plotters, in which reliability and simplicity are important attributes.
For such printers, a simple cutting mechanism such as that illustrated in FIG. 5 has been proposed. In this cutting mechanism, a blade 44 is drawn transversely in the direction of arrow 46 across the bed 48 that supports the paper sheet 50. The blade can be mounted on a carrier 52 that rides in a track 54 across the path of the paper. While such an arrangement clearly has the virtue of simplicity, it has a drawback not found in the guillotine-type cutter, namely, that the paper tends to bunch up ahead of the blade 44, as is suggested in FIG. 5 by paper region 56. Furthermore, it shares a further disadvantage with all of these arrangements, which is that the blade edge tends to become dull quite rapidly, thereby increasing maintenance--i.e., sharpening--costs or degrading cut quality if the sharpening is not performed frequently enough.
A further prior-art arrangement for paper cutting is depicted in FIGS. 6 and 7, in which two discs 58 and 60 rotate about parallel shafts 62 and 64. The upper disc 58 has a large-diameter central portion 66 between two smaller-diameter portions 68 and 70. The lower disc 60 has a smaller-diameter portion 72 between two larger-diameter portions 74 and 76 so that the peripheries of the two discs 58 and 60 are complementary. The discs 58 and 60 thus cut paper at the point at which the periphery of the center portion 66 of the upper disc 58 intersects the peripheries of the outer portions 74 and 76 of the lower disc 60. While the arrangement of FIGS. 6 and 7 also has a certain degree of simplicity, it also suffers from the tendency to cause paper to bunch and thus result in ragged cuts.
Accordingly, an object of the present invention is to obtain high and reliable cutting quality in a cutting mechanism that is relatively simple and thus reliable.